1. Field of the Invention
This invention relates to an electric power steering apparatus which assists the power required for the steering operation of an automobile.
2. Description of the Related Art
An electric power steering apparatus has been developed in which a steering torque acting on a steering wheel is detected, a driving current the value of which is set depending upon the detected torque is supplied to a DC motor for assisting the steering operation to drive it, so that the turning force of the DC motor assists the power required for the steering operation of an automobile, thereby providing a driver with a comfortable feeling of steering. In such an electric power steering apparatus, a driving circuit for the DC motor is composed of a bridge circuit consisting of four power transistors.
FIG. 1 is a schematic block diagram illustrating the configuration of a control system of a conventional electric power steering apparatus. In the figure, reference numeral 1 designates a DC motor for assisting the steering operation, and the DC motor 1 is driven by a motor driving circuit 2. The motor driving circuit 2 is composed of a bridge circuit consisting of two forward rotation power transistors 20a and 20b and two reverse rotation power transistors 20c and 20d which function as switching elements. The forward rotation power transistor 20a and reverse rotation power transistor 20d are connected in series, the reverse rotation power transistor 20c and forward rotation power transistor 20b are connected in series, and the two series circuits are connected in parallel. The DC motor 1 is connected between the node of the forward rotation power transistor 20a and reverse rotation power transistor 20d and that of the reverse rotation power transistor 20c and forward rotation power transistor 20b.
The collector of the forward rotation power transistor 20a and that of the reverse rotation power transistor 20c are connected to a main power supply 5 through a current detection resistor 8 and a fail-safe relay circuit 6. On the other hand, the emitter of the forward rotation power transistor 20b and that of the reverse rotation power transistor 20d are coupled to ground via a current detection resistor 9. The bases of the forward rotation power transistors 20a and 20b and reverse rotation power transistors 20c and 20d are coupled to a control unit 7. The fail-safe relay circuit 6 comprises a relay contact (not shown) which in its ON state supplies the electric power from the main power supply 5 to the motor driving circuit 2 and in its OFF state shuts off the supply of electric power. The operation of the relay contact is controlled by the control unit 7. The current detection resistor 8 is coupled to an over-current detection circuit 80 which detects a current flowing into the motor driving circuit 2 and supplies the detection result to the control unit 7. Similarly the current detection resistor 9 is coupled to a current detection circuit 90 which detects a current flowing through the DC motor 1 and supplies the detection result to the control unit 7.
The control unit 7 comprises a microcomputer and a PWM modulation circuit. The microcomputer performs a control of the degree of assisting the steering operation in which the degree of assisting the steering operation for the DC motor 1 is calculated on the basis of the detection results of a torque sensor and a speed sensor (both are not shown), and also a fail-safe control in which the relay contact of the fail-safe relay circuit 6 is made into the OFF state when an abnormality in the torque sensor, the speed sensor, the motor driving circuit 2, etc. is detected. The PWM modulation circuit generates a PWM output (hereinafter, referred to as "PWM signal") corresponding to the degree of assisting the steering operation which has been obtained by the microcomputer.
The operation of the thus configured electric power steering apparatus will be described. When the DC motor 1 is to be forward rotated, the PWM signal for controlling the motor which is outputted from the PWM modulation circuit is applied to the base of the forward rotation power transistor 20a, and a continuous signal for controlling the motor by which the forward rotation power transistor 20b is continuously turned on is applied to the base of the forward rotation power transistor 20b. In this case, therefore, each time when the forward rotation power transistor 20a is turned on in response to the PWM signal, a current flows through the main power supply 5, the fail-safe relay circuit 6, the current detection resistor 8, the forward rotation power transistor 20a, the DC motor 1, the forward rotation power transistor 20b and the current detection resistor 9, in this sequence, so that the DC motor 1 is driven to forward rotate. In contrast, when the DC motor 1 is to be reversely rotated, the PWM signal outputted from the PWM modulation circuit is applied to the base of the reverse rotation power transistor 20c, and a continuous signal for controlling the motor by which the reverse rotation power transistor 20d is continuously turned on is applied to the base of the reverse rotation power transistor 20d. In this case, therefore, each time when the reverse rotation power transistor 20c is turned on in response to the PWM signal, a current flows through the main power supply 5, the fail-safe relay circuit 6, the current detection resistor 8, the reverse rotation power transistor 20c, the DC motor 1, the reverse rotation power transistor 20d and the current detection resistor 9, in this sequence, so that the DC motor 1 is driven to reversely rotate. In the both cases, the driving force is regulated by the duty ratio of the PWM signal.
When a ground fault occurs for example in the DC motor 1 while a current flows through the DC motor 1, however, a large current flows through the DC motor 1 so that the assisting steering force of the degree which is greater than the driver's intention is exerted, resulting in a dangerous steering state. In this way, the flow of a large current caused by the ground fault of the DC motor 1 increases the current flowing into the motor driving circuit 2. In order to prevent a dangerous steering state due to this large current from occurring, the control unit 7 monitors the detection result of the overcurrent detection circuit 80 to determine whether or not it exceeds a given value. When the state wherein the detection result exceeds the given threshold threshold value continues for a predetermined period of time, the control unit 7 performs a fail-safe control in which the relay contact of the fail-safe relay circuit 6 is made into the OFF state so that the supply of electric power to the motor driving circuit 2 is cut off, thereby forcibly stopping the DC motor 1.
FIG. 2 is a graph illustrating the variation of current value which is detected by the overcurrent detection circuit 80 when a ground fault occurs in the DC motor 1. The graph in which detected current value is plotted as ordinate and the elapsed time as abscissa shows the relationship between them. In FIG. 2, a ground fault of the DC motor 1 occurs at time t.sub.1, and after the occurrence of the ground fault the current flowing through the DC motor 1 increases in the manner of time-lag of first order in accordance with the electrical time constant of the DC motor 1 and reaches the given threshold value A at time t.sub.4. When the current value continues to be equal to or greater than the given threshold value A for the predetermined period of time, i.e. at time t.sub.5, the fail-safe control is performed.
In a conventional electric power steering apparatus such as described above, however, a large current which is caused by a ground fault and flows through the DC motor 1 increases in the manner of time-lag of first order in accordance with the electrical time constant of the DC motor 1, and therefore it requires a considerable period of time for the current to reach the given threshold value A. When the detection of a ground fault consumes a period of time longer than required in this way, the period of time during which the assisting steering force of the degree which is greater than the driver's intention is exerted is extended correspondingly, thereby causing a problem in that a dangerous steering state continues for a long period of time.